The present invention relates generally to operations performed in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides a method of locating a cementing plug.
It is general practice in well cementing operations to use a top plug to separate cement pumped into a casing string from fluid used to displace the cement through the casing string. The fluid used to displace the plug, and thereby displace the cement, through the casing string is typically water, brine or mud.
A predetermined volume of the displacement fluid is pumped into the casing string on top of the plug. The volume of the displacement fluid pumped into the casing string on top of the plug is usually equal to the internal volume of the casing string. In this manner, the plug is pumped to the bottom end of the casing string, and the cement formerly in the casing string is displaced into an annulus formed between the casing string and the wellbore.
When the plug arrives at the bottom end of the casing string it lands in a float shoe or collar which is specially designed to receive the plug. After the plug lands in the float shoe, continued pumping of displacement fluid into the casing string will result in a noticeable pressure increase within the casing, thereby giving an indication that the plug has landed in the float shoe or collar.
Ideally, the plug lands in the float shoe at the same time as the predetermined volume of displacement fluid has been pumped into the casing string. Unfortunately, however, circumstances are not always ideal. For example, the plug may be released into the casing string late, that is, after the displacement fluid has already begun to be pumped into the casing string. In this situation, it would be preferred to stop pumping the displacement fluid before the plug lands in the float shoe, so that the displacement fluid introduced into the casing string before the plug was released will not be displaced into the annulus. As another example, due to pumping inefficiencies and/or other difficulties, the exact volume of displacement fluid pumped into the casing string may not be known, thereby making it difficult to know how far the plug has been displaced through the casing string.
From the foregoing, it can be seen that it would be quite desirable to provide a method which permits the location and/or displacement of a cementing plug in a casing string to be determined. It is accordingly an object of the present invention to provide such a method. Another object of the invention is to provide methods usable for other applications, as well.
In carrying out the principles of the present invention, in accordance with an embodiment thereof, a method is provided which utilizes pressure pulses transmitted during a cementing operation to determine a location and displacement of a plug in a casing string.
In one aspect of the invention, one or more pressure pulses are applied to the displacement fluid as it is being pumped into the casing string on top of the plug. If the speed of the pulse through the displacement fluid is known, then a difference in time between pulses reflected off of the plug will relate to a displacement speed of the plug. This information may be used to determine whether the plug is displacing in response to the displacement fluid being pumped into the casing string.
In another aspect of the invention, one or more pressure pulses reflected off of the plug may be used to accurately determine the location of the plug in the casing string, or to estimate the location. For the most accurate determination, a pressure pulse is transmitted through the displacement fluid a known distance, and the speed of the pressure pulse through the displacement fluid is determined before the displacement fluid is used to pump the plug through the casing string. Thus, when a pressure pulse is later transmitted through the displacement fluid and reflected off of the plug during the cementing operation, the speed of the pressure pulse through the fluid will be accurately known for use in calculating the location of the plug. For estimating the location of the plug, an approximate speed of the pressure pulse through the displacement fluid may be used.
In yet another aspect of the invention, the location of the plug may be estimated by transmitting a pressure pulse through the fluid in the casing string and counting reflections of the pressure pulse due to known anomalies in the casing string. For example, pressure pulse reflections due to collars in the casing string may be received prior to receiving a reflection due to the plug. By counting the number of the collar reflections prior to the plug reflection, the location of the plug in the casing string maybe estimated within the length of one joint of casing.
These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of a representative embodiment of the invention hereinbelow and the accompanying drawings.